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Reciprocating Pumps

Pumping Capacity   Example Question

BASIC PRINCIPLES

I. The pumping action in any reciprocating pump is dependent upon the positive displacement or the fluid pumped by a piston or plunger. The capacity of the pump is, therefore, determined by the area of the piston and its rate of travel. In order to obtain a practical machine, some method of reversing the direction of the pistons is required. In the direct-acting steam pump, this is accomplished by the steam valves and valve gear; in power pumps, this is accomplished by use of crank and connecting rods.

2. The ability of the pump to produce pressure is dependent upon the ratio of total steam force (steam pressure per unit area x area of steam piston) to total liquid force  (pump head x area of liquid piston). In order that pumping may occur, it is necessary that the steam force exceed the liquid force by an amount which slightly exceeds the various mechanical and hydraulic losses encountered. The basic principles for steam pump operation are shown below.

 

TYPES OF PUMPS

I. Direct-acting reciprocating pumps are classed as follows:

A. Horizontal or vertical.

B. Single or duplex. A single pump has one liquid piston or its equivalent single or double-acting plunger; a duplex pump has two liquid pistons or their equivalent single or double-acting plungers.

C. Single or double-acting. A single-acting unit pumps on one direction of piston travel only whereas double-acting units pump on both strokes. Direct-acting steam pumps are usually double acting.

2. Direct-acting steam pumps are conventionally described by stating the steam cylinder diameter, the liquid cylinder diameter, the length of stroke, horizontal or vertical (H or V), single or duplex (S or D), and single or double-acting (SA or DA). Thus a pump identified as 11 x 8 x 18

VSDA has an 11 inch steam cylinder, 8 inch water cylinder, 18 inch stroke, and is a vertical single double-acting pump.

 

General Characteristics

Reciprocating pumps move water or other liquids by a plunger or piston that travels back and forth inside a cylinder.

Positive displacement, often used for small capacities and when needed to avoid churning of centrifugal pumps. Can pump foaming liquids and high viscosity liquids.

Can control flow by regulating speed of drive with no head loss by throttling as in a centrifugal pump. Used often at high or very high pressures.  Also often used as metering pumps because of constancy of flow rate. The flow rate can be easily changed by adjusting the RPM of the driver.

Pumps ideally will produce any head that is impressed on them. The maximum head is determined by the power available and the strength of the pump parts. An automatic relief valve set at a safe pressure is used on the discharge side of all positive displacement pumps.

Never throttle on the discharge side to reduce the flow rate of a positive displacement pump. The fluid has no place to go and something will break. Can throttle on the steam driver or regulate the RPM of the electric motor to change the flow rate.

Unlike centrifugal pumps, positive displacement pumps are self priming.

 

USAGE IN MARITIME SERVICE

On ships a great number of applications are still served by steam reciprocating pumps, including:

A. Auxiliary feed.

B. Standby fuel oil service.

C. Fuel oil transfer.

D. Auxiliary circulating and condensate.

E. Fire and bilge.

F. Ballast.

G. High pressure evaporator.

H. Lubricating oil transfer.

I. Cargo stripping.

J. General service.

Direct-acting steam reciprocating pumps are not obsolete. If the steam conditions are not too severe in pressure, temperature, or superheat, they have many features of simplicity, reliability, and economy of operation and maintenance that still warrant serious consideration for many services.

Operation

PREPARING PUMP FOR OPERATION

The following steps .should be taken before putting a pump into operation for the first time, after an overhaul, or after the ship has been drydocked:

1. Check alinement and correct if necessary. If pump is operated out of line, scoring of rods and liners will result. 2. Steam and liquid lines should be free from scale and foreign matter.

3. Check all packing and repack if necessary.

4. Move steam pilot valve rods by hand to be sure pilot valve moves easily.

5. Check all connections and fittings to ensure they are tightly in place.

 

STARTING PROCEDURE

To start a reciprocating pump proceed as follows:

1. Oil the pins of the steam valve operating gear and set up on all grease cups.

2. Open the liquid end valves: a. Suction.

b. Discharge.

3. Open the cutout (or root) valves in the: a. Exhaust line. b. Steam line.

4. Open steam cylinder drains:

a. Top.

b. Bottom.

c. Valve chest.

5. Open exhaust valve at pump.

6. Crack the throttle valve and open slowly so as to ad- mit steam and warm up gradually.

7. Close the steam cylinder drains after the pump makes a few strokes and the steam cylinder is clear of water.

8. Bring the pump up to the proper speed by sufficiently opening the throttle valve. If pump is controlled by a pres- sure governor, open throttle gradually until governor takes control of pump and then open the throttle valve fully.

9. Adjust the cushioning valves, if fitted, until an adjust- ment is obtained that permits silent and smooth working of the pump, i.e., sufficient pump speed at the end of the stroke without knocking. After best point of operation is obtained, cushion valve should be set and not changed. When a reciprocating pump is operating at maximum speed, the cushioning valves should be almost completely closed.

STOPPING AND SECURING

To stop and to secure, proceed as follows:

1. Close the throttle valve.

2. Close the exhaust.

3. Open cylinder drains.

a. Top.

b. Bottom.

c. Valve chest.

4. Close the water end suction valve.

5. Close the water end discharge valve.

6. Close the steam and exhaust cutout valves (root valves).

7. After steam cylinder is drained, close the valve chest drains leaving the steam cylinder drain valves open to prevent hydraulic action.

 

Pumping Capacity  

Simplex single acting pumps discharge the cylinder volume for each 2 strokes.  The forward stroke discharges the cylinder and the back stroke or reverse stroke fills the cylinder.

Simplex double acting pumps discharge the cylinder volume for each pump stroke.  The forward stroke discharges the cylinder in front of the piston while filling the cylinder behind the piston.  The back or reverse stroke discharges the cylinder behind the piston while filling the cylinder forward of the piston.

Duplex double acting pumps use 2 double-acting cylinders in parallel, and pump two cylinder volumes for each pump stroke.

Duplex single acting pumps use 2 single-acting cylinders in parallel, and pump one cylinder volume for each pump stroke.

 

Capacity

Pump Capacity in GPM Gallons Per / Minute  =  volume discharged in gallons per pump stroke multiplied by strokes per minute.

 To determine the volume of the cylinder,  multiply the area of the circle by the height of the cylinder.

Volume of a Cylinder is equal to:

= (area of the circle) * (height)

= (p R2) * (height)

=  p R2 H

Example Question   USCG No.  3361

If you have a simplex double acting reciprocating pump making 110 strokes/minute, with a 5" diameter cylinder, a 4" stroke and operating with 95% volumetric efficiency, what is the capacity of this pump?

First we need to find the volume of the cylinder.

Pi The ratio of the circumference to the diameter of a circle = 3.14159265358979323846... = 3.14

Radius Squared = 6.25

Height = 4

(3.14  x 6.25) x 4 = 78.5 cubic inches

Double acting pumps discharge the cylinder volume on each stroke so we multiply 78.5 cubic inches by 110 strokes per minute. If this was a single acting pump which only discharges on the forward stroke we would divide the number of strokes in half.

78.5 cubic inches  x  110 strokes per minute  =  8635 Cu. In. per minute

Convert cubic inches to gallons, 1 gallon  = 231 cubic inches

8635 cubic inches divided by 231  =  37.38 gallons per minute

The question states the pump is operating with 95% volumetric efficiency.  Multiply the capacity by 95%

37.38 x .95 =  35.51 GPM

 

Note - Many of these questions have the wrong answer. If you have any of these questions on your exam make sure you put the question number next to your work, and save your scratch paper. If you receive a failing grade, you can check the answers.